The video signal samples supplied as output from a CCD imager are customarily a-c coupled to a d-c restoration circuit to be clamped to a prescribed direct potential during line retrace times. During the portion of line retrace that the CCD imager output signal is to be clamped, the output signal is at black level. The a-c coupling customarily is provided by resistive coupling in which the video signal samples pass through a d-c blocking capacitor to appear on the coupling resistance. The coupling resistance may be the input resistance of an amplifying device, for example. This coupling resistance is shunted by a relatively low-resistance clamping resistance during dc restoration. The RC time constant associated with the d-c blocking capacitor and the clamping resistance as divided by the duty factor of clamp, hereinafter referred to as the "clamping time constant", has been found to be a significant factor in controlling the visibility of frame-rate flicker.
Noise components at frequencies above the scan line rate have appreciable amplitude during the short time interval within the line retrace that black level clamping is done. Clamping to these noise components superposed on black level samples-and-holds these components to stretch their durations across full line scans. This gives rise to horizontal streak variations in average brightness across the televised picture, so-called "streak noise". It has been the practice to make the RC time constant many line times long, in order to reduce streak noise by integration; but this has an undesirable side effect associated with it. Frame-rate flicker becomes more noticeable when the clamping time constant is relatively long as compared to a line time.
Frame-rate flicker arises from sources of two general types, the inventor discerns. The first type is where the photoconversion efficiency in one set of alternate fields differs from that in the other set of alternate fields. This causes a difference in the corresponding charge packets in successive fields of static image samples with regard to the amplitudes of their photoresponse components. Frame-rate flicker of this type tends to be reduced by arranging for image integration intervals to be the same duration in all fields. In practice this is usually accomplished by doing two things. Firstly, field transfer times are made to take place at intervals exactly one field time apart, with all field times being of exactly the same length. Secondly, integration voltages applied to the image register gate electrodes in the sets of alternate fields are such as to cause collection efficiencies for all fields to be the same. In any case, this type of frame-rate flicker can be nulled by differentially adjusting gains in alternate fields, and in practice the null adjustment tends to be stable with time.
The second and less obvious type of frame-rate flicker arises from the actual black levels for the two sets of alternate fields differing from each other. The black level samples supplied by a CCD imager of field transfer or interline transfer type, during the periods of dc-restoration clamping, are generated by overscanning the output line register. That is, the output line register, the successive charge transfer stages of which are parallelly loaded with a scan line of charge packets, is subsequently forward clocked at the pixel scan rate. Forward clocking of the output line register proceeds for a long enough time to serially supply from its output port, not only the line of charge packets sampling image, but also an ensuing succession of empty wells that have been clocked across the entire length of the output register. The relatively short duration (53 microseconds or so in broadcast television standard operation) of the passage of an empty well across the line register allows scant time for the collection of accumulated dark current; and, of course, the empty well has no photogenerated charge component.
The black levels, on which the photoresponsive components of charge packets clocked on the CCD imager output stage during line trace are superposed, contain appreciable dark current. The accumulated dark current included in the black level accompanying each photoresponse has two components: one a direct component accumulated in the image register during image integration time, and the other a staircase component accumulated in the field storage register between image transfer and read out. The half line difference in the times alternate fields are stored in the field storage register causes slight (0.2% or so) differences in the dark current components of two fields. More significantly, the black levels in the CCD imager output signal are superposed on relatively large clocking signals. This clock feedthrough can be four times as large as full-well response. In 20 dB-less-than full-well response, a frame rate flicker of 10% or so will be a thousandth of clock feedthrough. So small differences in clock feedthrough on alternate fields can give rise to appreciable frame-rate flicker during the processing of output signal samples from the CCD imager. Such differences in clock feedthrough may be attributed to different static charge patterns on the image register in alternate fields. Frame-rate flicker of the second type described is elusive and difficult to combat in practice, which suggests that a mechanism such as that just discussed--i.e., one involving subtractive combinations of relatively large input signals to separate out a relatively small output signal--is in fact responsible for this persistent frame-rate flicker.
A long clamping time constant in the dc restoration circuit causes the black level of one field to persist into the next to exacerbate the frame-rate flicker, but a several line time constant is required to suppress streak noise. The inventor solves this dilemma by using a several line time constant in the dc restoration of each set of alternate field scans, but rapidly changing the black level to which dc is restored between successive field scans.
The problem of being able to rapidly adjust from one black level to another in different portions of a television signal is encountered in a different timing context in television systems which alternate color difference signals on a line-by-line basis. R. E. Flory in U.S. Pat. No. 4,414,572 issued Nov. 8, 1983 and entitled "CLAMP FOR LINE-ALTERNATE SIGNALS" describes dc-restoration circuitry using keyed clamping of a sort that permits independent restoration of dc levels on alternate sets of lines. Variants of the dc-restoration circuitry, differing from that specifically described by Flory can be constructed which allow independent restoration of dc levels in alternate fields.